1. Field of the Invention
The present invention relates to valves, and more particularly, to miniature, latching, low-power valves.
2. Discussion of the Related Art
Microelectromechanical structures (MEMS) and other microengineered devices are currently being developed for a variety of applications because of their size, cost and reliability. Many different varieties of MEMS devices and actuators have been created, including switches, valves, microgears, micromotors and other micromachined devices that are capable of motion or applying force. These MEMS devices may be employed in a variety of applications, including hydraulic applications in which MEMS pumps or valves are utilized, and optical applications in which MEMS light valves and shutters are utilized.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, cantilevers have been employed to apply mechanical force in order to rotate micromachined springs and gears. In addition, some micromotors are driven by electromagnetic fields, while other micromachined structures are activated by piezoelectric or electrostatic forces. MEMS devices that are actuated by the controlled thermal expansion of an actuator or other MEMS components have also been developed. These thermal actuators may comprise arched beams formed from silicon or metallic materials or combinations thereof that further arch or otherwise deflect when heated, thereby creating motive force. As an additional example of a type of thermally actuated device, thermal inkjet printing may be considered one of the classic applications of MEMS.
In practically every application of MEMS devices, precisely controlled and reliable movement is required. Given the micron scale dimensions associated with MEMS structures, stable and predictable movement characteristics are important. The movement characteristics of MEMS devices can be affected by intrinsic factors such as the type of materials utilized to fabricate the MEMS device, the dimensions and structure of the MEMS device, and the effects of semiconductor process variations. In addition, the movement characteristics of MEMS devices can be affected by extrinsic factors such as fluctuations in the ambient temperature in which the MEMS device operates. The impact of both the intrinsic and extrinsic factors may vary from device to device. For example, while thermally actuated MEMS devices are affected by all of the above factors, they are particularly sensitive to ambient operating temperature variations. Essentially, unless thermal compensation is built into the device or thermal control is incorporated as part of the device packaging, some types of thermally actuated MEMS devices may operate unpredictably or erroneously since the MEMS device will move not only in response to thermal actuation caused by active heating or cooling, but also due to changes in the ambient operating temperature. Therefore, it would be advantageous to develop other types of thermally actuated structures that would operate more reliably or more precisely even when exposed to significant ambient temperature fluctuations. Numerous applications, including switches, relays, variable capacitors, variable resistors, valves, pumps, optical mirror arrays and electromagnetic attenuators would be better served by MEMS structures with these attributes. However, thermal actuators are utilized when necessary.
An inexpensive, miniature, latching, low-power valve for the control of liquid flows is desired for many applications of high commercial interest, for example, drug delivery devices, including implantable medical devices. Prior art valves generally require complicated fabrication and require power to maintain the on or off state (open or closed). Many types of thermally actuated valves are inappropriate for control of liquid flows due to high power requirements because of the thermal conductivity of the liquid. For example, U.S. Pat. No. 5,058,856 describes a thermally actuated valve particularly well suited for application to gas flows, but not liquid flows.
A bubble or liquid/gas interface may be utilized to regulate liquid flow, as described in U.S. Pat. No. 6,062,681. Bubble valves can provide an attractive alternative to a mechanical valve. However, prior art bubble valves often require power, at least periodically, to maintain the off or closed position. In addition, if these valves are utilized to control flow against a significant pressure differential, the maximum flows attainable are often far less than those required in many applications. This is because it is difficult to hold a bubble in position against a significant pressure drop unless a very narrow opening is used to hold the bubble in place. If such a narrow opening is used, then the pressure required for adequate flow through the opening may exceed the system requirements. If, however, the pressure is raised, then this in turn will require a smaller opening to hold the bubble in place, reducing the flow for a given driving pressure, which can be a losing proposition.
A simplified equation for the pressure differential across an interface between a liquid and a gas is given byΔP=2σ cos θ/r,wherein ΔP is the change in pressure across the interface, σ is the surface tension of the liquid, θ is the interfacial angle between the liquid and an adjoining solid surface, and r is the radius of curvature of the vapor/liquid interface. A simple equation for volumetric flow rate in a cylindrical channel under conditions of laminar flow is given byQ=πΔPR4/8 μL,wherein ΔP is the pressure differential driving the flow given above, R is the radius of a circular channel, μ is the viscosity of the fluid, and L is the channel length. Even in an application in which the required flow rate is relatively low, such as a medical application, the limitations due to the governing physics can be problematic. For example, in a medical device requiring a flow rate of 1 ml/week, having a thirty-five mm channel length, utilizing an aqueous fluid and a driving pressure of twenty to fifty PSI, the opening needed to maintain a bubble in position would be about an order of magnitude smaller than the overall channel size required.
Accordingly, there exists a need for an accurate, reliable, inexpensive, miniature, latching, low-power valve for the control of liquid flows in a wide variety of applications, including drug delivery devices.